The above-noted patent discloses a planar, bilateral switching integrated circuit comprising four vertical NPN transistors sharing a common collector, with leads between the emitters and bases of adjacent pairs. The effective circuit, shown in FIG. 1 of the accompanying drawings, is a lateral triac comprising eight transistors, the NPN's noted above and four horizontal PNP's sharing a common base, in a transistor bridge construction.
Because all junctions reach a single surface, the device can be made photosensitive. In the basic configuration it is provided with a pair of gate terminals. When a gate current is applied between one of the gate terminals and an associated main terminal, or when a light-generated photocurrent reaches a certain magnitude, the device is triggered. Because of the symmetrical construction, AC circuits may be controlled, e.g. a single gate current can trigger the device in both guadrants I and III.
We call this new device a "photrac", since it is best described as a photosensitive triac, and as a symbol therefore we have adapted the well-known symbol for a silicon-controlled rectified (SCR), but with two gate terminals and arrows which indicate photosensitivity. ##SPC1##
In FIG. 1, the NPN transistors are labelled Q1 through Q4, and the PNP's are Q5 through Q8. Light lines indicate internal interconnections inherent in the planar construction, and heavy lines indicate external leads. Either gate, when biased to its respective main terminal, can trigger conduction in quadrants I and III; this allows control from either or both sides of an AC line. The typical size of such a device is 25 mils square.
As also disclosed in the patent, it is preferred to diffuse the base in two stages. One stage is a normal base diffusion, and the other, which is through a larger mask so as to surround the first base, is two to four orders of magnitude lower in impurity concentration, and no higher than about 10.sup.16 atoms/cc. This improves voltage characteristics, raising breakdown voltage well above normal AC line voltages. The method of high voltage diffusion is described in U.S. Pat. No. 3,920,493 issued Nov. 18, 1975, and devices including same are disclosed in U.S. application Ser. No. 416,170 filed Nov. 15, 1973, both assigned to the same assignee as the instant application.